Broadband contrast polarizing glass

ABSTRACT

A polarizing glass article, and a method of making the article, that exhibits a broad band of high contrast polarizing properties in the infrared region of the radiation spectrum, that is phase-separated by precipitating silver, copper, or copper-cadmium halide crystals in the glass within a size range of 200-5000 Å, and that contains elongated silver, copper, or copper-cadmium metal particles formed on or in the halide crystals, and having an elongated aspect ratio of at least 2:1, the article having a contrast ratio of at least 100,000 over a range of at least 300 nm.

This application is a 371 of PCT/US97/21227, filed Nov. 18, 1997, andclaims benefit of provisional application 60/032,390, filed Dec. 04,1996.

FIELD OF THE INVENTION

A polarizing glass produced from a phase-separated glass containingsilver, copper, or copper-cadmium halide crystals and a method ofproduction.

BACKGROUND OF THE INVENTION

A polarizing effect can be generated in glasses containing silver,copper, or copper-cadmium halide crystals. These crystals can beprecipitated in aluminosilicate glasses having compositions containingsuitable amounts of an indicated metal and a halogen other thanfluorine.

The polarizing effect is generated in these crystal-containing glassesby stretching the glass, and then exposing its surface to a reducingatmosphere. The glass is placed under stress at a temperature above theglass annealing temperature. This elongates the glass, and therebyelongates and orients the crystals. The elongated article is thenexposed to a reducing atmosphere at a temperature above 250° C., but notover 25° C. above the glass annealing point. This develops a surfacelayer in which at least a portion of the halide crystals are reduced toelemental silver or copper (hereafter “metal”).

The production of a polarizing glass, then, involves, broadly, thesefour steps:

1. Melting a glass batch containing a source of silver, copper, orcopper-cadmium and a halogen other than fluorine, and forming a bodyfrom the melt,

2. Heat treating the glass body at a temperature above the glass strainpoint to generate halide crystals having a size in the range of 200-5000Å,

3. Stressing the crystal-containing glass body at a temperature abovethe glass annealing point to elongate the body and thereby elongate andorient the crystals, and

4. Exposing the elongated body to a reducing atmosphere at a temperatureabove 250° C. to develop a reduced surface layer on the body thatcontains metal particles with an aspect ratio of at least 2:1.

The growth of halide particles cannot occur at temperatures below thestrain point of the glass because the viscosity of the glass is toohigh. Higher temperatures, above the annealing point, are preferred forcrystal precipitation. Where physical support is provided for the glassbody, temperatures up to 50° C. above the softening point of the glasscan be employed.

The production process is described in detail in U.S. Pat. No. 4,479,819(Borrelli et al.). There it is pointed out that the halide crystalsshould have a diameter of at least about 200 Å in order to assume, uponelongation, an aspect ratio of at least 5:1. When reduction to elementalmetal particles occurs, the particles having an aspect ratio of at least5:1 will display an aspect ratio greater than 2:1. This places the longwavelength peak at least near the edge of the infrared region of theradiation spectrum, while avoiding serious breakage problems during thesubsequent elongation step. At the other extreme, if the diameter of theinitial halide particles exceeds about 5000 Å, significant haze developsin the glass. This is accompanied by a decreased dichroic ratioresulting from radiation scattering.

The dichroic ratio is a measure of the polarizing capability of a glass.It is defined as the ratio existing between the absorption of radiationparallel to the direction of elongation and the absorption of radiationperpendicular to the direction of elongation. To attain an adequateratio, the aspect ratio of the elongated halide crystals must be atleast 5:1 so that the reduced metal particles have an aspect ratio of atleast 2:1.

Crystals having a small diameter demand very high elongation stresses todevelop a necessary aspect ratio. Also, the likelihood of glass bodybreakage during a stretching-type elongation process is directlyproportional to the surface area of the body under stress. These arevery practical limitations on the level of stress that can be applied toa glass sheet, or other body of significant mass. In general, a stresslevel of about five thousand psi has been deemed to be a practicallimit.

The literature indicates that firing of the elongated body in a reducingatmosphere should be undertaken at temperatures above 250° C., but nohigher than 25° C. above the annealing point of the glass. A reductiontemperature as high as is compatible with the tendency for crystals torespheriodize is desirable. The time required decreases dramaticallywith increase in temperature. In particular, there is an abrupt changein the time required to achieve complete reduction above 400° C., thatis, above the melting temperature of the metal halide phase. It isthought, although not clearly proven, that the metal from the halidephase grows considerably faster when the halide phase is molten. Thisexperimental fact means that, to carry out the reduction treatment in apractical time interval, requires a temperature above 400° C.,preferably above 415° C. Looking at the phenomenon in another way, inorder to produce, in a reasonable time, a depth of reduced layernecessary for a high contrast, the reduction treatment must be carriedout at a high temperature.

One of the key measures of the effectiveness of a polarizing glass bodyis its contrast ratio, or, as referred to in the art, contrast. Contrastcomprises the ratio of the amount of radiation transmitted with itsplane of polarization perpendicular to the elongation axis to the amountof radiation transmitted with its plane of polarization parallel to theelongation axis. In general, the greater the contrast, the more useful,and valuable, the polarizing body.

Another important feature of a polarizing body is the bandwidth overwhich the body is effective. This property takes into consideration notonly the degree of contrast, but the portion of the spectrum withinwhich the contrast is sufficiently high to be useful. A contrast ratioof 100,000 has been taken as a point of reference for comparisonpurposes. Clearly, the lower the reference contrast, the broader thecorresponding bandwidth. We have chosen 100,000 (50 db) because itrepresents a common high performance value specified for polarizerapplications.

The peak contrast wavelength is determined by the aspect ratio of theelongated particle. The aspect ratio increases with the degree of stressapplied to stretch the glass, and thereby the crystals. The wavelengthat which the peak contrast occurs increases with the aspect ratio. Mostapplications in the infra-red require a peak in the wavelength range of1300-1550 nm. However, other applications require contrast peaks outsidethis range, for example, as low as 600 nm.

Heretofore, it has been necessary to produce polarizing glass articleson an individual basis. Thus, it was necessary to design a separate setof processing conditions tailored to provide the peak contrast for eachapplication wavelength. Then care had to be taken to control the processquite rigidly. The particle elongation is controlled by controlling theelongating stress applied.

The maximum bandwidth available heretofore has been about 300 nm, with acommercially practical figure being no more than 200 nm.

For example, an article might be designed having a center wavelength(CWL), that is, a contrast peak, at about 900 nm. The article would,however, have an optimum bandwidth of about 200 nm covering the range of800-1000 run. As a result, the article would not be effective atwavelengths outside this range, e.g. 1240, 1310 and 1560 nm.

It would, of course, be highly desirable to provide a polarizing glasshaving a much broader bandwidth of contrast ratios above the practicaluse level that is now available. Ideally, this would extend from thevisible into the infrared portions of the spectrum.

It is then a basic purpose of the present invention to meet this need.Another purpose is to provide a polarizing glass that is effective overa broad range of wavelengths. A further purpose is to provide a singlepolarizing glass article that is broadly useful in a variety ofapplications. A still further purpose is to provide a method of makingsuch a polarizing glass article.

SUMMARY OF THE INVENTION

The invention resides in a polarizing glass article that exhibits abroad band of high contrast polarizing properties in the infrared regionof the radiation spectrum, that is phase-separated by precipitatingsilver, copper, or copper-cadmium halide crystals in the glass within asize range of 200-5000 Å, and that contains elongated silver, copper, orcopper-cadmium metal particles having an elongated aspect ratio of atleast 2:1 and formed on or in the halide crystals, the article having acontrast ratio of at least 100,000 over a range of at least 300 nm.

The invention further resides in a method for making a glass articleexhibiting a relatively broad band of high contrast polarizingproperties in the infrared region of the radiation spectrum from glasseswhich are phase-separable to form silver, copper, or copper-cadmiumhalide crystals, the method comprising the steps of:

(a) melting a batch for a glass containing a source of silver, copper,or copper-cadmium and at least one halogen other than fluorine,

(b) cooling and shaping the melt into a glass article of a desiredconfiguration,

(c) subjecting the glass article to an elevated temperature for a periodof time sufficient to generate and precipitate silver, copper, orcopper-cadmium crystals in the glass, the crystals ranging in sizebetween about 200 and 5000 Å,

(d) elongating the glass article under stress at a temperature above theannealing point of the glass to elongate the crystals and align them inthe direction of the stress, and,

(e) exposing the elongated glass article to a reducing atmosphere at atemperature above about 250° C., but below about 400° C. to initiatereduction, to silver or copper metal, of spots on, or in, the halideparticles to form nuclei, and conducting the reduction at a pressure ofat least 10 atmospheres for a period of time sufficient to develop areduced surface layer on the glass article within which the nuclei aregrown into particles of varying aspect ratio deposited in and/or uponsaid elongated crystals, the aspect ratio being at least 2: 1, wherebythe glass article exhibits a relatively broad range of high contrastpolarizing properties in the infrared region of the radiation spectrum.

Prior Art

Prior literature of possible interest is listed and described in anattached document.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation showing the contrast ratio curvefor a polarizing glass article produced in accordance with priorconventional practice.

FIGS. 2-5 are graphical representations of typical contrast ratio curvesobtained with the present invention.

DESCRIPTION OF THE INVENTION

The present invention adopts, and improves on, the known method ofproducing a polarizing glass body. Basically, it embodies the steps ofmelting a glass containing a source of silver, copper, or copper-cadmiumand one or more halogens other than fluorine, forming a body from theglass, and cooling. It further embodies the conventional steps of heattreating the glass body to form and precipitate halide crystals ofsilver, copper, or copper-cadmium and then heating and subjecting thebody to stress to elongate the halide crystals. In accordance withconventional practice, the body is then subjected to a thermal reductionstep, preferably in a hydrogen atmosphere, to reduce a portion of thesilver or copper halide crystals in a surface layer on the body toelongated metal particles having an aspect ratio of at least 2:1.

Practice of the present invention contemplates employing all of thesteps in the conventional manner without changes, except for the finalreduction step. The present invention is concerned only with the finalstep in which reduction of metal halide, to metal takes place. In abroad sense, it is proposed to carry out the reduction step at a lowertemperature below 400° C. and at a high pressure. This produces reducedmetal particles of a different nature, and that have a different effecton polarizing characteristics.

As indicated earlier, present practice produces a polarizing glass witha relatively narrow bandwidth. Bandwidth is determined by thedistribution of elongated particles that result after hydrogen reductionof the stretched glass. In particular, it is the summation of the aspectratios of the particle shapes. Each shape produces a peak contrast at adifferent wavelength. The shape of a contrast versus wavelength curvefor a polarizing glass is therefore the superposition of the peaks forall the particles.

The aspect ratio of the crystal particles is a function of elongatingstress. Consequently, the contrast peak and bandwidth shift across theinfrared spectrum depending on the elongating stress. For example, thevalues for a polarizer effective at 1500 nm are quite different from oneeffective at 600 nm. With the reduction of the halide to metal inaccordance with conventional practice, the aspect ratio changes, but thedistribution remains essentially the same.

The present invention is based on a way of producing a broaderdistribution of metal particle aspect ratios using the same initialhalide crystal distribution. It has been observed that reduction of thehalide crystals to the metal state occurs very slowly at temperaturesbelow 400° C. It appears that, in order to obtain reduction within areasonable time under normal practice, it is necessary that the halidebe molten. Silver halide melts at 400° C.

The reduction process is pictured as occurring by formation of metalnuclei at spots on, or in, the halide particles. Growth of the nucleithen occurs, but at a very slow rate below 400° C. While ultimatecomplete reduction of a halide particle would be expected to occur, ithas not been observed to occur within any practical time at atemperature below 400° C.

We have now found that the rate of reduction can be greatly increased ata temperature below 400° C. by operating at pressure markedly above thenormal one atmosphere. While some effect is achieved at a pressure onthe order of 10 atmospheres, it is preferred to operate at 50-100atmospheres, and even higher if practical. We have found that thereduction rate varies as the square root of the pressure. Also, thereaction proceeds with a dependence on the square root of time.Consequently, by employing a reduction pressure of 100 atmospheres at agiven temperature, the time required at one atmosphere is reduced by afactor of 100. This then provides a practical reduction process at atemperature below 400° C.

The significance of this discovery is not simply the ability to operateat a temperature below 400° C. Rather, it is the ability to achieve amuch broader bandwidth than heretofore attainable. This is due to thefact that metal particles grown from nuclei on, or in, the metal halideparticles have a different shape and aspect ratio from that of thecrystal itself. As a result, there are, effectively, a range of otheraspect ratios added to the available distribution. This in turn providesthe desired broader bandwidth as measured at a contrast of 100,000.Thus, with pressure of 100 atmospheres of hydrogen, we can obtainbandwidths of 700-900 nm, as compared to the commercial value of 200 nmheretofore available.

The glass employed may be any of the known glasses that can bephase-separated to form silver, copper, or copper-cadmium crystals inthe glass. Such glasses are disclosed, for example, in U.S. Pat. Nos.4,190,451 (Hares et al.) and 3,325,299 (Araujo) disclosing photochromicglasses and 5,281,562 (Araujo et al.) disclosing non-photochromicglasses. Each of these patents is incorporated by reference,particularly for its teaching of glass composition ranges and theirproduction. Preferred glasses are those disclosed in the Hares et al.patent.

It is, of course, necessary to form halide crystals of silver, copper,and/or copper-cadmium in the glass article. This may occur duringcooling. However, the preferred practice is to cool quickly, and thenreheat under controlled conditions to precipitate the necessarycrystals. It has been customary to perform the steps at a temperaturebelow 750° C. However, a companion application, provisional no.60/027,256 filed Sep. 30, 1996 in the names of D. G. Grossman et al.,describes a method characterized by heating at a temperature of 750° C.or higher, preferably for at least an hour. This provides variousadvantages as described in that application.

As indicated, the glass containing halide crystals, must be elongated tostretch and orient the crystals. This prepares the crystals for furthertreatment to prepare them for reduction to produce a polarizing glass.Conventional practice is to conduct this step at about 710° C.

The present invention is concerned with, and modifies, the final step inwhich the glass is subjected to a thermal reduction treatment. Inaccordance with prior practice, the thermal reduction treatment wascarried out at a temperature on the order of 415° C. for times of 3-6hours and at a pressure of one or two atmospheres. It was considereddesirable to employ as high a temperature as compatible with thetendency to respheriodize.

In contrast, the production treatment step of the present invention iscarried out at a temperature below 400° C. and at a high pressure. Whilesome improvement may be obtained at pressures of 5-10 atmospheres, it ismore practical to operate at a higher pressure, for example, 100atmospheres reducing gas pressure. The maximum pressure is dependent onthe capability of the chamber employed.

As explained earlier, this modified reduction treatment permitsachieving high contrasts over a much broader bandwidth. Our preferredpractice, then, is to achieve contrasts greater than 100,000 over abroad bandwidth by exposing the glass to a reducing gas, preferablyhydrogen at as high a pressure as practical for a period of one hour ata temperature of 350-380° C.

The time of treatment will depend on the depth of reduction layerdesired. While the depth is not critical, we prefer a depth of about 100m. At temperatures of 350-380° C., this may be obtained in a time ofabout one hour.

A reducing atmosphere of H₂ is most effective. However, this may bediluted for safety considerations, and other known reducing atmospheresmay also be employed.

The procedure just described is effective to increase bandwidth acrossthe infra-red spectrum. However, it is most effective at lowerwavelengths of 600-1200 nm.

We have further found that the effect at longer wavelengths can befurther enhanced by a subsequent treatment at a temperature above 400°C., for example at 415° C. This produces a much shallower reduced layerof about 10-15 m. Strangely enough, the two reduced layers appear tooperate independently and do not have a detrimental effect on eachother. As a result, the order of treatment is not important. However, itis usually more convenient to conduct the lower temperature treatmentfirst.

The invention is further described with reference to test pieces ofglass processed in identical manner, except for the hydrogen atmosphereconditions employed during the reduction step. Data obtained frommeasurements on the test pieces after the reducing treatments areplotted in the accompanying drawings. The glass employed in making testpieces to obtain the data presented in the drawings has the followingcomposition in % by weight as calculated from the batch on an oxidebasis:

SiO₂ 56.3 ZrO₂ 5.0 B₂O₃ 18.2 TiO₂ 2.3 Al₂O₃ 6.2 Ag 0.24 Na₂O 5.5 CuO0.01 Li₂O 1.8 Cl 0.16 K₂O 5.7 Br 0.16.

FIG. 1 is a graphical representation in which contrast ratios areplotted on the vertical axis. Wavelengths in nm are plotted on thehorizontal axis. The glass test piece employed in this test wasstretched at a temperature in the range of 580-610° C. in accordancewith commercial practice for attaining a peak central wavelength of 1300nm. It was then exposed to a hydrogen atmosphere at one atmospherepressure for four hours at 420° C.

The curve in the drawing is based on contrast ratios of the twocomponents of polarized light as measured between about 800 and about1500 nm. The horizontal, dashed line shows the wavelength range overwhich the contrast ratio is over 100,000. The breadth of this range isabout 200 nm between 1200 and 1400 nm.

FIG. 2 is a corresponding graphical representation of data measured onthe test piece of FIG. 1 after a subsequent treatment. This treatmentwas carried out for 1 hours at 350° C. in a hydrogen atmosphere at apressure of 100 atmospheres.

As in FIG. 1, contrast ratios are plotted on the vertical axis andwavelengths in nm on the horizontal axis. Likewise, the horizontal,dashed line shows the wavelength range over which the contrast ratiosare above 100,000 nm. The breadth of this range is about 700 nm andextends between about 700 and about 1400 nm. It is evident that thetreatment of the present invention greatly expands the breadth of therange at the 100,000 ratio, as well as extending it down to lowerwavelengths. Thus, this polarizer would be effective for use ateffective wavelengths of 900, 1100 and 1300 nm.

Similar tests were carried out on comparable test pieces that werestretched at a somewhat higher stress to provide a CWL of about 1480 nm.This produced a breadth of about 240 nm between 1360 and 1600 nm withone test piece subjected to the standard one atmosphere hydrogenpressure at 420° C. Treatment with 100 atmospheres at 350° C. produced abreadth of about 900 nm between about 600 and 1500 nm on the other testpiece.

FIG. 3 is a graphical representation corresponding to FIGS. 1 and 2, butshowing data measured on another test piece. This test piece wasstretched under a stress adapted to produce a CWL of about 900 nm, andreceived only a single thermoreduction treatment. This treatment was ata temperature of 350° C. for 1 hours with a pressure of 100 atmosphereshydrogen. The curve in the FIG., like that in FIG. 1 is based oncontrast ratios of the two components of polarized light measured atwavelengths from 600 to 1700 nm. The dotted line shows the breadth ofthe wavelength band at a contrast ratio of 100,000. The value is about600 nm from 600 to 1200 nm.

It will be appreciated that the specific embodiments merely illustrate,rather than limit the invention. Thus, wavelength bands for a contrastratio of 100,000 may be obtained at different wavelengths by varying thestretching stress.

FIGS. 4 and 5 are further graphical representations corresponding toFIGS. 1-3. They show contrast versus wavelength curves for test piecestreated under different conditions.

The test piece represented by FIG. 4 was heated in a hydrogen atmosphereat a pressure of 100 atmospheres for 16 hours at 280° C. While a bitlong to be commercially practical, this data illustrates theeffectiveness of the invention at a low temperature approaching theminimum temperature of about 250° C. The bandwidth is about 500 nm.

The test piece of FIG. 5 shows the result of reducing a test piece for apresent commercial time under a pressure of 100 atmospheres of hydrogenand a temperature of 350° C. This demonstrates that the bandwidth ofabout 200 nm, obtainable by conventional practice, can be extended to900 nm, a four to five fold increase.

We claim:
 1. A polarizing glass article that exhibits a broad band ofhigh contrast polarizing properties in the infrared region of theradiation spectrum, that is phase-separated by precipitating silver,copper, or copper-cadmium halide crystals in the glass within a sizerange of 200-5000 Å, and that contains elongated silver, copper, orcopper-cadmium metal particles having an elongated aspect ratio of atleast 2:1 and formed on or in the halide crystals, the article having acontrast ratio of at least 100,000 over a range of at least 300 nm.
 2. Apolarizing glass article in accordance with claim 1 having a contrast inexcess of 100,000 over a bandwidth of at least 300 nm.
 3. A polarizingglass article in accordance with claim 2 wherein the bandwidth is in therange of 400-900 nm.
 4. A method for making a glass article exhibiting abroad band of high contrast polarizing properties in the infrared regionof the radiation spectrum from glasses which are phase-separable to formsilver, copper, or copper-cadmium halide crystals, the method comprisingthe steps of: (a) melting a batch for a glass containing a source ofsilver, copper, or copper-cadmium and at least one halogen other thanfluorine, (b) cooling and shaping the melt into a glass article of adesired configuration, (c) subjecting the glass article to an elevatedtemperature for a period of time sufficient to generate and precipitatesilver, copper, or copper-cadmium crystals in the glass, the crystalsranging in size between about 200 and 5000 Å, (d) elongating the glassarticle under stress at a temperature above the annealing point of theglass to elongate the crystals and align them in the direction of thestress, and, (e) exposing the elongated glass article to a reducingatmosphere at a temperature above about 250° C., but below about 400° C.to initiate reduction, to silver or copper metal, of spots on, or in,the halide particles, and conducting the reduction at a pressure of atleast 10 atmospheres for a period of time sufficient to develop areduced surface layer on the glass article within which the nuclei aregrown into particles of varying aspect ratio deposited in and/or uponsaid elongated crystals, the aspect ratio being at least 2:1, wherebythe glass article exhibits a broad range of high contrast polarizingproperties in the infrared region of the radiation spectrum.
 5. A methodin accordance with claim 4 which comprises exposing the article to apressure of at least 50 atmospheres.
 6. A method in accordance withclaim 4 which comprises exposing the article at a temperature of atleast 280° C.
 7. A method in accordance with claim 4 which comprisesexposing the article for at least one hour.
 8. A method in accordancewith claim 4 which comprises further exposing the article at atemperature above 400° C. but not over 25° C. above the glass annealingpoint.